This invention relates to metallic seals and seal components which are energized in a novel manner, which are damage resistant and which can be made smaller than conventional metal-to-metal seals. Although for illustrative purposes the invention is discussed below largely in the context of completions for oil and gas wells, it is applicable to metallic seals in general.
There is a trend towards subsea completions incorporating increasingly large bores. Current subsea xmas tree system configurations (both parallel and concentric) can be inefficient in terms of space usage within the tubing hanger assembly. For large completion bore systems it would be advantageous to reconfigure the subsea xmas tree system whilst maintaining a large number of down-hole lines through the tubing hanger. A solution for releasing additional radial space to facilitate larger completion bores would be to reduce the size of the mechanism for sealing off the annulus void.
The design of large bore subsea xmas trees and completions is constrained due to requirements of utilizing existing standard BOP configurations. Therefore, in order to run larger completion tubing, space must be saved elsewhere to permit using existing BOP""s. Additionally, particularly in the case of deepwater developments, significant cost savings can be achieved by using smaller standard BOP and casing programs while still maintainingxe2x80x94or increasingxe2x80x94the radial space available for the completion tubing. In this way vessel selection is made easier, and hence costs decreased, due to smaller handling requirements associated with the smaller BOP size.
The problematic situation of a drive toward larger bore completions coupled with potentially utilizing smaller BOP stacks makes the radial space taken within the well system for annular packoffs of prime importance. Any space saved here can have a direct impact on the size of the completion tubing that can be accommodated.
Essentially, the sealing requirement for a slick bore tubing hanger is to seal the annulus between the tubing hanger and spool (wellhead, xmas tree or tubing spool), maintaining a clearance while running in the hanger, and once the hanger is in position, energizing the seal to a set (sealed) condition. In the particular case of horizontal production outlet tubing hangers, it is usual to seal the annulus above and below the horizontal outlet. In the case of conventional tubing hangers (or casing hangers), only one seal barrier is required to seal off the annulus.
The prior art is replete with descriptions of seal systems involving a metal sealing element that bears against a metallic surface to establish a metal-to-metal sealing interface preventing the passage of corrosive or non-corrosive pressurized fluid throughout a wide temperature and pressure range. Packoffs providing metal-to-metal seals are disclosed for example in U.S. Pat. Nos. 4,900,041 and 5,174,376. Both patents disclose annular metal seal elements having a generally U-shaped cross-section. An energizing mandrel is driven downwardly between the two legs of the U-shaped seal profiles thereby deflecting the outer leg outwardly. However the energizing mandrel and its associated actuating mechanism is often relatively large, leading to a bulky packoff assembly. Furthermore, the range of movement imparted to the seal element outer leg by the mandrel is restricted, meaning that the sealing surface or area within the wellhead housing or other spool bore with which the seal element outer leg co-operates must closely surround the seal element in its relaxed or non-energized state. Such seal areas/surfaces therefore cannot be recessed for protection during drilling operations, so remaining exposed and vulnerable to damage.
In accordance with the present invention, a seal assembly comprises an annular sealing surface and a metallic sealing element comprising in an unenergized state a thin-walled, substantially cylindrical tube or sleeve having first and second opposed ends, the sealing element in use being compressed so as to move the opposed ends towards one another, such movement causing a center section of the sealing element between the ends to bulge transversely of their direction of relative movement and sealingly engage the sealing surface. By this means relatively large deflections of the element center section can be obtained.
Such a sealing element has a wide variety of applications. It may be used to seal between a bore and a member, such as a shaft, received within the bore to isolate pressure in either direction across the sealing element.
To permit the necessary kinking and bulging, the length to wall thickness ratio of the tube is preferably greater than about 4. However, to avoid unstable or asymmetric buckling, the length to diameter ratio of the tube should not be too great, preferably no larger than about 2.5. A preferred length to wall thickness ratio is about 15. The particular dimensions most appropriate to a given sealing application will vary from case to case and can be determined experimentally. The sealing element may also include geometrical features that initiate the kinking and bulging in the required manner. For example, in the case of a generally tubular sealing element, the end faces may be shaped so as to produce higher compressive loading near the bore than towards the circumference of the sealing element. Additionally or alternatively, the bore may be widened towards the center section of the sealing element.
The invention also provides a metallic sealing element comprising a hollow center section whose interior is filled with a relatively easily deformable, relatively incompressible substance, the center section comprising walls of V-shaped cross-section or of concertina or bellows-shaped cross-section; the sealing element comprising first and second opposed ends which in use are compressed so as to move towards one another, such movement causing the V-shaped or concertina or bellows-shaped center section walls to bulge transversely of the direction of relative movement of the ends and sealingly engage a co-operating sealing surface.
A seal assembly including the sealing element, a bore and a member received in the bore, may be formed in which the member and/or the bore comprise(s) a pair of bodies having respective faces against which the respective ends of the sealing element are received, the bodies being movable towards each other to cause the bulging of the sealing element center section. Such movement may be generated by any of the means used to actuate conventional metal-to-metal seals into the energized or set condition, for example weight applied to one of the bodies, adjustment nuts or similar wedging means, or hydraulic piston and chamber actuators.
The member or bore preferably comprises a recessed sealing surface with which the sealing element center section makes sealing contact. Because it is recessed, the sealing surface is protected against mechanical damage and erosion.
The invention is useful not only in sealing the tubing annulus of a well completion, but also annuli at other well tubulars, such as casing hangers. The member may therefore be a tubing or casing hanger carrying the sealing element, with the bore formed in a spool in which the hanger is landed. The faces referred to above may both be provided on the hanger, or alternatively one face may be provided on the hanger and the other on the spool.
The hanger may have a lockdown mechanism that is also arranged to cause the relative movement between the faces for energizing the sealing element. No separate sealing element energizing actuator is therefore necessary, leading to a particularly compact sealing assembly.
The sealing element center section, or the sealing surface with which it cooperates, may comprise an insert or coating of relatively soft metal, adapted to form a better seal.
In its preferred embodiments, the invention fulfils one or more of the following objectives:
1. It provides reliability under cyclical loading, with good protection of co-operating seal surfaces against erosion and accidental damage.
2. It can be remotely operated using simple tooling or simply set by tubular string weight and/or a hanger lockdown mechanism.
3. It accommodates 10,000 psi (69 MNmxe2x88x922) nominal maximum working pressure as a typical base case. However, a family of such sealing assemblies may be produced, also including, for example, members for 5,000 psi (35 MNmxe2x88x922), 15,000 psi (104 MNmxe2x88x922) and other applications as required.
4. It is effective over a temperature range of at least 0xc2x0 F. to 250xc2x0 F. (xe2x88x9217.8xc2x0 C. to 121xc2x0 C.) and preferably beyond at either end.
5. It provides a compact seal assembly.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings.